This invention relates to fetal probes and, more particularly, to a fetal probes having improved fetal contact.
During in utero labor and delivery fetal probes are used to monitor health of the fetus. Typical parameters monitored include arterial blood oxygen saturation levels, pulse rate, heart rate and temperature. Pulse oximeters are typically used to measure various blood characteristics including arterial blood oxygen saturation and pulse rate. An electrocardiogram (ECG) is used to monitor heart rate. A thermal sensor is used to monitor temperature. In order to achieve accurate measurements, the aforementioned sensing devices must maintain good contact with the fetus during parameter measurement.
Improper contact to the fetus with a pulse oximetry probe may result in inaccuracies caused by shunting of light between the emitter and detector. FIG. 1 shows a transflectance pulse oximetry probe 110 comprising a light emitter 114 and a light detector 118 disposed within a housing 122. If the probe 110 is in poor contact with the skin 134, (e.g. due to extraneous material 138 such as hair, mucous, etc., between the probe and skin 134), light may be directly reflected from the top surface of the skin or piped through the extraneous matter as shown by path 2. Alternatively, the light may scatter below the surface of the skin, but may not travel deep enough to penetrate the blood perfused layer 130. Instead, the light travels through bloodless layer 132 as shown by path 3.
The depth uniformity and/or location of layer 132 is negatively affected by local vasoconstriction, excessive force applied to the back surface of the probe (which locally exsanguinates blood from the tissue beneath the probe), and site-to-site variations of the distance to the blood perfused layer. The aforementioned factors negatively affect blood oxygen saturation measurements. Unless probe placement is well controlled in an environment free of excessive applied force and other light shunting causes, accuracy of the calculated saturation will be suspect.
To improve contact to the fetus, the fetal probe described in pending U.S. patent application Ser. No. 07/775,315, filed Oct. 11, 1991, incorporates a fixed curved sleeve and an oppositely curved removable stiffener. The stiffener is inserted in the probe to make the handle substantially straight during insertion of the sensor housing. When the stiffener is removed, the curved sleeve presses the sensor housing against the fetus.
Similarly PCT Application No. WO 91/07910 discloses a fetal probe which has an inflatable bladder connected to the non-sensing side of the probe. The bladder has a hemispherical or spherical shape in order to provide a wedging action which will maintain the position of the fetal sensor during contractions, or fetal or maternal movements. Although the addition of an inflatable bladder improves fetal contact, unequal forces are applied to different contact points of the fetus. In addition, because of the position of the bladder, there are site-to-site variations between the sensor and the blood perfused layer. An improved method for contacting an oximeter probe to a fetus is needed.